Metal surface inspection device for metal rings of a continuously variable transmission belt

ABSTRACT

A metal surface inspection device for metal rings of a Continuously Variable Transmission (CVT) V-belt which does not perform excessive detection of mackerel skins as a detrimental manufacturing flaw. This task is achieved by providing an “integral value calculation portion” for calculating an integral value of a difference value and a “difference value correction portion” for correcting the difference value in a decreasing direction as the integral value increases. When mackerel skins contained on an inspectable surface are detected the output (integral value of the difference value) of the integral value calculation portion increases. Accordingly, the difference value is corrected in a decreasing direction which enables the operation to control the signal of the mackerel skin portions and threshold values are not exceeded which prevents excessive detection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal surface inspection device. More particularly, the present invention relates to a metal surface inspection device suitable for use in inspecting the surface of metal rings which are parts that constitute a V-belt type of a Continuously Variable Transmission belt (hereinafter referred to as “CVT belt”).

2. Description of the Related Art

Conventionally, there is a known CVT belt structure which laminates a plurality of thin metal rings in a stack of approximately 0.2 mm in thickness to which steel elements are consecutively attached.

FIG. 5 is an outline view of a CVT belt. In this diagram, a CVT belt 1 is constructed by assembling two laminated bands of a layered belt 2 that contain a stack of a number of metal rings 2 a (for example, a laminated band composed of about 12 endless layers) which are supported by a layered element 3 composed of a large number of steel elements 3 a (for example, about 400 elements).

In this manner, the structure of the CVT belt 1 is produced through the following processes:

-   -   (1) First, a ring-shaped drum is formed by welding together the         ends of a thin sheet of ultrahigh strength steel, such as         maraging steel.

(2) Next, the drum is cut into round slices of a predetermined width and rolled to create metal rings 2 a of a basic peripheral length.

(3) Next, after performing a solution treatment, etc. to the above-mentioned metal rings 2 a, the necessary peripheral length (namely, the peripheral difference between the inner and outer periphery) corresponding to the stacked layers of the CVT belt 1 using a “peripheral length correction device” is accomplished. Furthermore, aging treatment, nitride treatment, etc. are performed to increase the hardness of the metal rings 2 a.

(4) Lastly, the metal rings 2 a are laminated after undergoing the above-mentioned process (3), the steel elements 3 a are consecutively attached and the CVT belt 1 is completed.

Naturally, since these metal rings 2 a undergo the above-mentioned processes (such as manufacture of the drum, cutting, rolling, solution treatment, peripheral length correction, aging treatment, nitride treatment, etc.), partial defects occur, such as abrasions and indentations on the front and rear end faces of the metal rings 2 a.

As an inspection method for such defects, there is a process for factory workers to determine the existence of abrasions, indentations, etc. by visually observing the front and rear end faces of the metal rings 2 a preceding the CVT belt manufacturing process (4), namely by directly viewing parts or using a magnifying glass. However, in this antiquated method there is the drawback of being inefficient due to the fact that human error rate is always higher than an automated process. Thus, satisfactory reproducibility and reliability are not routinely acquired.

As for conventional prior art which is applicable to the surface inspection of the above-mentioned metal rings 2 a, for example, Japanese Laid-Open Patent Application No. H11-248637 (1999) titled “DEFECT DETECTING DEVICE” (hereinafter denoted as “conventional prior art device”) is known.

Particularly noteworthy, the principle configuration of this conventional prior art device comprises an inspection light source which illuminates the inspectable surface and at least two light guiding paths (optical fiber) which guide reflected light from the inspectable surface to the light reception segments, and the spacing arrangement of the two light guiding paths is separated at a slight distance. Thus, in such a configuration when an inspectable surface does not have a defect, etc., the reflected light guided by the two light guiding paths is supplied to the light reception segments at substantially the same intensity. On the other hand, when an inspectable surface has a minor defect, since there is a decline (light intensity decline by diffused reflection) in the reflected light of an applicable defective part, a difference occurs in the light of the light guiding paths and the existence of a defect can be automatically discriminated from the amount of this difference.

However, although the above-mentioned conventional prior art is a beneficial device from the viewpoint of being able to automatically discriminate whether or not a defect exists on an inspectable surface, there is a drawback in which the inspection process excessively detects for example “mackerel skins” as manufacturing defects which do not impair the durability of the metal belt.

Here, “mackerel skins” is a common name referring to an abrasion with gloss which appears continuous or with a certain amount of length in the circumference direction of a metal belt. In nearly all cases, the entire abrasion is a smooth surface and the flaw does not impair the durability of a metal belt. Consequently, this type of flaw can be ignored as having no impact on the finished product. Thus, it is a waste of resources to exclude metal rings which have such a mackerel skin as a defective part and not preferred in terms of manufacturing cost.

Therefore, the object of the present invention is to provide an inspection device for metal rings which does not perform excessive detection of a mackerel skin as a detrimental flaw.

SUMMARY OF THE INVENTION

A metal surface inspection device for metal rings of a Continuously Variable Transmission (CVT) V-belt related to the present invention comprises a light source for illuminating an inspectable surface of the metal ring to be inspected; a first light guiding path for guiding a reflected light from the inspectable surface to a first light detector and a second light guiding path for guiding the reflected light to a second light detector; a difference value calculation means for calculating a difference value between an electrical signal outputted from the first light detector or an electrical signal correlated to its electrical signal and an electrical signal outputted from the second light detector or an electrical signal correlated to its electrical signal; and a discrimination means for discriminating the existence of a defect on the inspectable surface and comparing the difference value with a predetermined threshold value, which is provided with an integral value calculation means for calculating an integral value of the difference value and a difference value correction means for correcting the amount of the difference value based on the integral value. The difference value correction means corrects the difference value in a decreasing direction when the integral value increase exceeds a predetermined value and the discrimination means discriminates the existence of a defect on an inspectable surface and compares the difference value corrected in the difference value correction means with a predetermined threshold value.

According to the present invention, excessive detection of mackerel skins is prevented by the following principle. In particular, (A) since mackerel skins on an inspectable surface are glossy abrasions, there is relatively little diffused reflection on the mackerel skin surface. Therefore, strong reflected light is observed on a mackerel skin surface; (B) since mackerel skins are flaws that are continuous or with a certain amount of length, the above-mentioned “strong reflected light” is observed and maintained only during the time period corresponding to the flaw length; and (C) relative to excessive detection of mackerel skins, a difference value increases by the above-mentioned “strong reflected light” which exceeds a threshold value.

In the present invention when the above-mentioned (A) and (B) correspond and an integral value increases exceeding a predetermined value, by providing a “difference value correction means” for correcting the difference value in a decreasing direction which corresponds to the above-mentioned (C), the result of these countermeasures prevents excessive detection of mackerel skins.

Briefly, detection of a mackerel skin on an inspectable surface increases the output (integral value of the difference value) of the integral value calculation means. Because the difference value in connection with this is corrected in a decreasing direction, ultimately the signal of a mackerel skin is suppressed and a threshold value is not exceeded (avoiding excessive detection).

The above and further objects and novel features of the present invention will more fully appear from the following detailed description when the same is read in conjunction with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual line block diagram of an inspection device of a metal ring;

FIG. 2 is a conceptual line block diagram of an inspection section 14;

FIG. 3 is a block diagram of a judgment section 40;

FIGS. 4A, 4B, and 4C are cross sectional plan views of a metal rings 2 a and waveform diagrams of an output signal (difference value Sd) from a difference calculation section 45; and

FIG. 5 is an outline view of a CVT belt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will hereinafter be described in detail with reference to the drawings. Additionally, in the following explanation of specific or examples of various details, numerical values or character strings and other illustrative symbols are merely references to clarify the concept of the present invention. Accordingly, the concept of the present invention should not be limited explicitly to this terminology entirely or in part.

In addition, explanation is omitted which describes details of well-known methods, well-known procedures, well-known architecture, well-known circuit configurations, etc. (hereinafter referred to as “common knowledge”) for the purpose of concise explanation, but does not intentionally exclude this common knowledge entirely or in part. Therefore, relevant common knowledge which is already known by persons skilled in the art at the time of filing the present invention is included in the following description.

FIG. 1 is a conceptual line block diagram of an inspection device of a metal ring. An inspection device 10 comprises a fixed position drive pulley 12 which is rotary driven by a motor 11, a variable position driven pulley 13 which is separately situated on the same rotational plane as the drive pulley 12 and an inspection section 14.

When examining the metal rings 2 a which are the object to be inspected, first, the driven pulley 13 is positioned at an initial position (refer to position “A” on the dashed dotted line). Subsequently, the metal rings 2 a are wound around the two pulleys (drive pulley 12 and driven pulley 13). Next, the desired tension is applied to the metal rings 2 a by supplying load W which has a predetermined mass (for example, 80 kg) and drives the driven pulley 13. In the state of operating the motor 11 which causes the metal rings 2 a to rotate unidirectionally (the direction of arrow “B”) and with the inspection section 14, a front and rear end face inspection is performed.

FIG. 2 is a conceptual line block diagram of the inspection section 14. Referring to this drawing, the inspection section 14 comprises at least two optical sensor sections 20 and 30 (hereinafter referred to as “A system optical inspection section 20” and “B system optical inspection section 30”, or simply “A system 20” and “B system 30”) and a judgment section 40. The reason for comprising “at least two” the optical sensor sections 20 and 30 is described later.

The A system 20 and B system 30 have the same configuration. Namely, the A system 20 (the B system 30) configuration includes two illuminating optical fibers 22 and 23 (32 and 33) for the purpose of guiding the light from a light source 21 (31) in parallel to the inspectable surface (here, although assumed as the “front end face” of the metal rings 2 a, it may be the “rear end face”) of an object to be inspected (the metal rings 2 a); alight reception optical fiber 24 (34) inserted between the illuminating optical fibers 22 and 23 (32 and 33); and a light detector 25 (35) which converts reflected light Pa (Pb) into an electrical signal Sa (Sb) from the inspectable surface guided with the light reception optical fiber 24 (34). The light reception optical fiber 24 constitutes “a first light guiding path” mentioned earlier in the summary of the present invention and a light detector 25 which constitutes “a first light detector” also mentioned in the above summary. Additionally, the light reception optical fiber 34 constitutes “a second light guiding path” and a light detector 35 constitutes “a second light detector” both mentioned in the summary of the present invention.

The judgment section 40 judges whether or not a flaw exists on an inspectable surface of the metal rings 2 a based on the electrical signal Sa outputted from the light detector 25 of A system 20 and the electrical signal Sb outputted from the light detector 35 of the B system 30. The basic principle as also described in the opening patent document 1 pertains to “the intensity of the light which enters into the two light detectors 25 (35) is substantially the same when an inspectable surface does not contain a defect and differs when there is a defect”. Also, “the difference value of the electrical signals Sa and Sb outputted from the two light detectors 25 (35) is acquired. When this difference value is greater, this is indicative that the inspectable surface contains a defect and will be discriminated”.

In other words, when an inspectable surface of the metal rings 2 a does not have a defect, the inspectable surface is a smooth surface and the light from the illuminating optical fibers 22 and 23 (32 and 33) is equally reflected in terms of being smooth and diffused reflection is hardly generated. Accordingly, the intensity of the light which enters into the light detectors 25 (35) is composed of the appropriate strength and substantially the same amount. In this case, the difference value of the electrical signals Sa and Sb are practically set to “0”.

On the other hand when the inspectable surface of the metal rings 2 a has a defect, the light from the illuminating optical fibers 22 and 23 (32 and 33) will reflect diffusely at the defective spot. Thus, the intensity of the light guided to the light detector 25 (35) via the light reception optical fiber 24 (34) only decreases by the amount of diffused reflection. In this case, the spacing of the A system 20 and the B system 30 is separated only by distance L. If this distance L is suitably greater than the above-stated defect size, when the light reception optical fiber of one system (for example, the light reception optical fiber 24 of A system 20) guides light declined in strength by the influence of a defect, the light reception optical fiber (light reception optical fiber 34 of B system 30) of the system on the other side will guide light not declined in strength (namely, intense reflected light strength from a smooth surface without a defect). Consequently, in this case, because the electrical signal Sa becomes less than (<) the electrical signal Sb, the difference value clearly becomes greater as compared with the above-mentioned normal condition (Sa=Sb).

The above principle can be applied as in “when the electrical signals Sa and Sb are outputted from the two light detectors 25 (35), the difference in values is calculated and a greater difference indicates an inspectable surface has a defect which can be discriminated”.

The reason at least two systems (the A system 20, the B system 30) are required is as follows: based on the above-stated principal explanation, when an inspectable surface does not contain a defect, the electrical signal Sa (or Sb) outputted from either of the systems constitutes a “greater value.” Subsequently, when an inspectable surface contains a defect, while either system is receiving reflected light (declined light strength only by the percentage of diffused reflection) from a defect, the electrical signal Sa (or Sb) outputted from that system constitutes a “lesser value.”

In the above principle, a judgment is possible by recognizing these “greater values” and “lesser values.” However, the surface of the metal rings 2 a used in a CVT belt as an object to be inspected is in most cases delustered (dull finish) and because the degree of delustering is not standard for each product (or lot), variations occur in the “greater value” of the electrical signal Sa (or Sb) which serves as the standard for normal judging. The influence of the above-stated variations can be eliminated by configuring the optical sensor sections with “at least two systems” and taking the “difference value” between the electric signal Sa (and Sb) outputted from those systems.

FIG. 3 is a block diagram of the judgment section 40. Referring now to this drawing, the judgment section 40 configuration includes an amplifier 4 l for A system, an amplifier 42 for B system, an AGC circuit 43 for A system, an AGC circuit 44 for B system, a difference calculation section 45 (difference value calculation means), a mackerel skin correction section 46, a high side threshold value judgment section 47 (discrimination means), a low side threshold value judgment section 48 (discrimination means), and an alarm signal generation section 49.

The amplifier 41 for A system amplifies the electrical signal Sa which is outputted from the light detector 25 of the A system and fluctuation control of the amplification factor is performed by the output of the AGC circuit 43 for A system. The AGC circuit 43 for A system includes a low-pass filter 50 which extracts only a low-frequency component contained in the continuous current from among the output signals of the amplifier 41 for A system and a differential amplifier 51 which generates the AGC voltage of the amount corresponding to the difference between the output of the low-pass filter 50 and a predetermined reference voltage REF1. The amplifier 41 for A system amplifies the electric signal Sa by the amplification factor corresponding to this AGC voltage. The purpose of this AGC voltage is to remove low-frequency component “fluctuations” (generated in connection with “surface blurring” of the metal rings 2 a) contained in the electrical signal Sa.

The amplifier 42 of B system like the above-stated amplifier 41 for the A system amplifies the electrical signal Sb outputted from the light detector 35 for the B system and fluctuation control of the amplification factor is performed by the output of the AGC circuit 44 for B system. The AGC circuit 44 for the B system includes a low-pass filter 52 which extracts only a low-frequency component contained in continuous current from among the output signals of the amplifier 42 for B system and a differential amplifier 53 which generates the AGC voltage of the amount corresponding to the difference between the output of the low-pass filter 52 and a predetermined reference voltage REF1. The amplifier 42 for B system amplifies the electrical signal Sb by the amplification factor corresponding to this AGC voltage. The purpose of this AGC voltage is the same as that above which is to remove low-frequency component “fluctuations” contained in the electrical signal Sb.

The difference calculation section 45 calculates a difference value Sd between an electrical signal Sa_41 outputted from the amplifier 41 for A system and an electrical signal Sb_42 outputted from the amplifier 42 for B system.

The mackerel skin correction section 46 is an integral part which is essential in the present invention. However, initially a case “when not provided” with the mackerel skin correction section 46 will be explained. Then, after describing its drawback, the configuration of the present invention (a configuration which does provide the mackerel skin correction section 46) will be explained.

First, the high side threshold value judgment section 47 compares the difference value Sd with a predetermined high side threshold value SL_H and outputs a high side determination result signal Sc_H which becomes active when Sd is greater than SL_H (Sd>SL_H). The low side threshold value judgment section 48 compares the same difference value Sd with a predetermined low side threshold value SL_L and outputs a low side determination result signal Sc_L which becomes active when Sd is greater than SL_L (Sd>SL_L). In addition, the alarm signal generation section 49 outputs an alarm signal ALM which indicates defect detection on an inspectable surface when either of these two determination result signals (Sc_H, Sc_L) become active.

FIG. 4A is a cross sectional plan view of the metal rings 2 a which has a mackerel skin 56 and a flaw 57 contained on part of that mackerel skin 56. FIG. 4B is waveform diagram of the output signal (difference value Sd) of a difference calculation section 45 in the case of not providing the mackerel skin correction section 46.

In FIG. 4B, the difference value Sd is composed of an abnormal region 60 in which the difference value Sd exceeds two threshold values (SL_H and SL_L) and a normal region 61 which does not exceed either of the threshold values. The abnormal region 60 is a signal corresponding to a mackerel skin 56 (flaw 57 contained on part of the mackerel skin 56) contained on a surface of the metal rings 2 a. The mackerel skin 56 as mentioned above is an abrasion with gloss which appears continuous or with a certain amount of length in the circumference direction of the metal rings 2 a. Also, the signal of the abnormal region 60 maintains a high level throughout almost the entire region. Therefore, the result readily exceeds the two threshold levels (SL_H and SL_L) and involuntarily due to the excessive detection, the mackerel skin 56 will be identified as a defect.

Further, as shown in the sectional view of FIG. 4A, a flaw 57 is contained on part of the mackerel skin 56. Since this flaw 57 penetrates deep into the inner part of the metal rings 2 a, it is a defect which cannot be ignored with regard to the durability of the metal rings 2 a and it is a flaw which must be applicable to detection. In the wave form diagram of FIG. 4B, since the peaks of the signal waveforms 58 and 59 corresponding to such a flaw 57 are observed and these signal waveforms 58 and 59 are also greater than the two thresholds (SL_H and SL_L), it is detectable as the defect.

However, with the buildup of the signal level accompanying a mackerel skin 56 because the entire area of the abnormal region 60 exceeds the two thresholds (SL_H and SL_L), it is undistinguishable whether or not detection of that defect is based on the mackerel skin 56 or according to the flaw 57 contained on a part of the mackerel skin 56.

Consequently, the inventors of the present invention have focused on the difference in the signal level along a time axis between the signal waveforms of the entire abnormal region 60 corresponding to the mackerel skin 56 and the signal waveforms of the other normal region 61. Specifically, the signal level of the former signal waveforms 60 (signal waveforms of the entire abnormal region corresponding to a mackerel skin 56) is continually monitored for high points relative to the latter signal waveforms (signal waveforms of the normal region 61). The signal level of the abnormal region 60 is decreased to a proper amount by calculating an integral value of the signal level and correcting the difference value Sd using this integral value. In this manner, when there is a flaw 57 contained on part of the mackerel skin 56, that flaw 57 is detected without difficulty. Simultaneously, the overload of the two threshold values (SL_H and SL_L) is avoided and excessive detection of a mackerel skin 56 is prevented.

Accordingly, the distinctive characteristic of the configuration in the present invention as shown in FIG. 3, the mackerel skin correction section 46 comprises a difference value amplifier 46 a, a full-wave rectification section 46 b, an integral value calculation section 46 c and a differential amplifier 46 d. Specifically, in the subsequent stage of the difference calculation section 45, the mackerel skin correction section 46 contains the difference value amplifier 46 a (difference value correction means) which utilizes a variable amplification factor for amplifying the difference value Sd. The full-wave rectification section 46 b accomplishes full-wave rectification of the bipolar difference value Sd extracted from the difference value amplifier 46 a which is then transformed into a unipolar difference value Sd. The integral value calculation section 46 c (integral value calculation means) calculates an integral value of the difference value Sd after full-wave rectification. The differential amplifier 46 d (difference value correction means) generates correction voltage for the amount corresponding to the difference of the output by the integral value calculation section 46 c and a predetermined reference voltage REF2.

The amplification factor of the difference value amplifier 46 varies corresponding to the amount of the correction voltage extracted from the differential amplifier 46 d. When the integral value calculated by the integral value calculation section 46 c exceeds a predetermined value (reference voltage REF2), the differential amplifier 46 d generates correction voltage corresponding to the excess portion. Though the integral calculation section 46 c calculates the integral value of the difference value Sd (after full-wave rectification) which passed through the difference value amplifier 46 a, this integral value is equivalent to the signal level added (integral) to the signal waveforms along a time axis in FIG. 4B for example. Accordingly, for example among the signal waveforms in FIG. 4B, this integral value constitutes a greater value in the abnormal region 60 and a lesser value in the normal region 61.

Therefore, in normalizing the reference voltage REF2 of the differential amplifier 46 d, when (for example, when the abnormal region 60 is observed) an integral value greater in level exceeding this reference voltage REF2 is acquired, the amplification factor of the difference value amplifier 46 a is managed in a decreasing direction. In this manner, the signal level of the abnormal region is controlled which avoids exceeding the two threshold values (SL_H and SL_L) and excessive detection of the abnormal region 60 (mackerel skins) can be prevented.

Additionally, when there is a flaw 57 contained on a part of a mackerel skin 56, only the peak signal waveforms 58 and 59 accompanying this flaw 57 exceed the two threshold values (SL_H and SL_L). Thus, while preventing excessive detection of a mackerel skin 56, a flaw 57 contained on a part of a mackerel skin 56 can be detected without difficulty.

FIG. 4C is a waveform diagram of the output signal (the difference value Sd) of the difference calculation section 45 in the case of providing the mackerel skin correction section 46 (according to the configuration of the present invention). In this waveform diagram, the difference value Sd is composed of the abnormal region 60 including the peak signal waveforms 58 and 59 corresponding to a flaw 57 contained on a part of a mackerel skin 56 and the other normal region 61. Also shown is how much control of the signal level is achieved over the entire abnormal region 60. Only the peak signal waveforms 58 and 59 corresponding to a flaw 57 contained on a part of a mackerel skin 56 exceed the two threshold values (SL_H and SL_L). Thus, while preventing excessive detection of a mackerel skin 56, a flaw 57 contained on a part of a mackerel skin 56 can be detected without difficulty.

The mackerel skin correction section 46 is provided in the subsequent stage of the difference calculation section 45 and the integral value after full-wave rectification of the difference value Sd is calculated by the mackerel skin correction section 46. When the integral value is greater than a predetermined value (reference voltage REF2), the difference value Sd is corrected in the decreasing direction. Naturally it is also necessary to determine the reference voltage REF2 according to the circumstances by experimenting.

Based on the above configuration, a flaw 57 contained on a part of a mackerel skin 56 can be detected as a defect without performing excessive detection of a mackerel skin 56 and an alarm signal ALM can be outputted. As a direct result, the metal rings 2 a which has only a mackerel skin 56 will pass inspection, significantly help reduce wasted material and lower the manufacturing costs.

Furthermore, the present invention is not limited to the above-stated embodiment. Within the scope of the technical concept, naturally various modifications or future development cases are included. For example, the present invention may be adapted as follows:

Although, in the above-mentioned embodiment, the inspectable surface is expressed as the “front end face” of the metal rings 2 a, it may be expressed as “the rear end face” of the same ring.

Moreover, in the above-mentioned embodiment, although the light from the light source 21 (31) are guided to an inspectable surface through the illuminating optical fibers 22 and 23 (32 and 33), they are not restricted to this example. The light from the light source 21 (31) may directly irradiate an inspectable surface or irradiate a surface via an optical element, such as an optical lens.

Besides, in the above-stated embodiment, although the optical sensor section is formed by two systems (the A system 20 and the B system 30), it may be configured with multiple systems exceeding two. Also, when configured with multiple systems more than two, each system may be situated on the periphery direction and width direction of the metal rings 2 a (a two-dimensional array).

Lastly, in the above-mentioned embodiment, although the reflected light from an inspectable surface is guided to the light detector 25 (35) via the light reception optical fiber 24 (34), the use of this “optical fiber” merely indicates the best mode of the embodiment. In brief, what is necessary is just a “light guiding object” which can guide reflected light from an inspectable surface to the light detector 25 (35) with the smallest possible intensity loss. For example, if intensity loss and flexibility are disregarded or ignored, the light guiding object may simply be made out of glass or plastic.

While the present invention has been described with reference to the preferred embodiments, it is intended that the invention be not limited by any of the details of the description therein but includes all the embodiments which fall within the scope of the appended claims. 

1. A metal surface inspection device for metal rings of a Continuously Variable Transmission (CVT) V-belt comprising: a light source for illuminating an inspectable surface of said metal rings to be inspected; a first light guiding path for guiding a reflected light from said inspectable surface to a first light detector and a second light guiding path for guiding said reflected light to a second light detector; a difference value calculation means for calculating a difference value between an electrical signal outputted from said first light detector or an electrical signal correlated to its electrical signal and an electrical signal outputted from said second light detector or an electrical signal correlated to its electrical signal; and a discrimination means for discriminating the existence of a defect on said inspectable surface and comparing said difference value with a predetermined threshold value, which is provided with an integral value calculation means for calculating an integral value of said difference value and a difference value correction means for correcting the amount of said difference value based on said integral value; said difference value correction means corrects said difference value in a decreasing direction when said integral value increases exceeding a predetermined value; and said discrimination means discriminates the existence of a defect on said inspectable surface and compares said difference value corrected in said difference value correction means with a predetermined threshold value.
 2. The metal surface inspection device according to claim 1, wherein said integral value calculation means calculates said integral value of a signal after accomplishing full-wave rectification of said difference value. 